The invention relates to a process for preparing ethylene oxide (called EO hereinafter) by direct oxidation of ethylene and to a process according to which glycol is produced from EO by hydrolysis, pressure dehydration, vacuum dehydration and subsequent distillation.
Currently, EO is prepared industrially by direct oxidation of ethylene with air or oxygen in the presence of silver catalysts. The reaction is highly exothermic (overall heat production from 225 to 400 kJ per mole of ethylene); therefore, to dissipate the excess heat of reaction, conventionally tube-in-shell reactors are employed, the reaction mixture being conducted through the tubes and a boiling liquid, for example kerosene or tetralin, recently frequently water, being circulated as heat carrier between the tubes. The present invention relates to processes according to which water is used as heat carrier.
Processes of this type are described, for example, in Ullmanns Encyclopedia of Industrial Chemistry, Fifth edition, Vol. A 10, pages 117ff. According to this, ethylene and oxygen are charged into a circulating gas stream which, in addition to the reactants, comprises inert gases and the byproduct of total oxidation of ethylene, carbon dioxide.
The water vapor produced in the direct oxidation of ethylene is, in the known process, generally expanded via a valve to the pressure of a steam grid. In the course of this, the energy content of the water vapor from the expansion is not utilized.
A significant proportion of ethylene oxide (EO) of the worldwide production is increasingly further processed to monoethylene glycol. To improve the selectivity of the EO hydrolysis, the hydrolysis reactor is operated with a high water excess (weight ratio of water: EO to 15:1). As a result, the proportion of higher glycols, in particular of diethylene glycol, triethylene glycol etc., can be forced down. The hydrolysis reactor is customarily operated at temperatures from 120xc2x0 C. to 250xc2x0 C. and pressures of 30-40 bar. The hydrolysis product is firstly dehydrated to a residual water content of 100-200 ppm and then fractionated into the various glycols in pure form.
The dehydration is generally performed in a cascade of pressure-staggered towers with decreasing pressure. For reasons of thermal integration, generally only the bottoms reboiler of the first pressure tower is heated with fresh steam, all further pressure towers in contrast are heated with the vapors of the respective preceding tower. Depending on water content of the hydrolysis reactor discharge and the pressure/temperature level of the external steam used in the bottoms reboiler of the first tower, the pressure dehydration cascade consists of from 2 to 7 towers. The pressure dehydration is followed by a vacuum dehydration. The dehydrated glycol-containing solution is fractionated in a plurality of towers into the pure substances monoethylene glycol, di- and triethylene glycol. It is an object of the present invention to utilize energetically, to the optimum extent, the water vapor produced in the direct oxidation of ethylene oxide with use of water as heat carrier and to improve the economic efficiency of the process for preparing EO and/or monoethylene glycol.
We have found that this object is achieved by a process for preparing EO by direct oxidation of ethylene with air or oxygen using water as heat carrier, with water vapor being formed which is then expanded. The invention comprises carrying out the expansion of the water vapor in one or more backpressure steam turbine(s).
Steam turbines describe in a known manner heat engines having rotating moving parts in which the pressure drop of constantly flowing steam is converted into mechanical work in one or more stages. Depending on the type of steam removal, various types of steam turbines are differentiated; in what are termed the backpressure steam turbines, the exhaust steam energy is further exploited for other purposes, generally for heating.
For the use in the present process, in principle any backpressure steam turbine can be used.
Steam turbines are generally operated with steam feed under constant conditions. In contrast thereto, according to the invention a steam turbine is operated with continuously increasing steam rate and rising steam pressure. The conditions (steam rate, pressure) in the time average are critical for economic efficiency of this solution.
The industrial catalysts used in the ethylene oxidation, which generally comprise up to 15% by weight of silver in the form of a finely particulate layer on a support, lose activity with increasing operating time, and the selectivity of the partial oxidation of ethylene oxide decreases. In order to keep the production rate of an EO plant constant with increasing operating time, the reaction temperature must be increased with the same conversion rate, as a result of which the pressure of the resulting water vapor increases. The simultaneous decreasing selectivity leads to greater steam rates. In the case of conventional industrial plants, frequently at the start of operation a water vapor pressure in the range of 30 bar arises, and with continuous increase over an operating time of 2 years to a value of about 65 bar.
Depending on the energy available, the steam turbine(s) can drive one or more working machines, in particular process pumps (to transport circulating water) or compressors (for gaseous process streams) and/or one or more generators. Water vapor fed to the steam turbine(s) generally has a pressure from 25 to 70 bar, preferably from 30 to 65 bar.